What I remember from my engineering school (as much I wanted to remember from
those "Mechanical" classes. I belonged to the elite: The electric and
electronic groups. It didn't help that much when it came to maintain a job
nowadays though :(.)
Anyhow, in order for the worm gear to be "self-braking" the efficiency of that
gear had to be less than 50%. It has to do with the angle of the screw thread
and the friction coefficient. A warning is in place: When the surfaces on the
gear and the worm start to wear, the friction coefficient tend to be reduced
and a gear box, that initially was self braking could later lose that
characteristic. You therefor had to take notice each time you use a worm gear
as it suddenly could "break lose".
73 de,
Hans - N2JFS
-----Original Message-----
From: Jon Pearl - W4ABC <jonpearl@tampabay.rr.com>
To: Patrick Greenlee <patrick_g@windstream.net>; towertalk
<towertalk@contesting.com>
Sent: Fri, Apr 4, 2014 5:04 pm
Subject: Re: [TowerTalk] Fulton 2500 winch single or dual speed?
Hi Patrick,
Not being a mechanical engineer, I knew what I wanted to say in response
to you, but I didn't know how to express it.
It's not the ratio, but the way the gear teeth mesh that determines
whether or not there will be spur gear 'creep'.
From http://acmegear.com/worm.htm
Worm gears are used when large gear reductions are needed. It is common
for worm gears to have reductions of 20:1, and even up to 300:1 or
greater. The worm can easily turn the gear, but the gear cannot turn
the worm. This is because the angle on the worm is so shallow that when
the gear tries to spin it, the friction between the gear and the worm
holds the worm in place.
A worm is a gear that resembles a screw. It is a species of helical
gear, but its helix angle is usually somewhat large (i.e., somewhat
close to 90 degrees) and its body is usually fairly long in the axial
direction; and it is these attributes which give it its screw like
qualities. A worm is usually meshed with an ordinary looking,
disk-shaped gear, which is called the "gear", the "wheel", the "worm
gear", or the "worm wheel".
The prime feature of a worm-and-gear set is that it allows the
attainment of a high gear ratio with few parts, in a small space.
Helical gears are, in practice, limited to gear ratios of less than
10:1; worm gear sets commonly have gear ratios between 10:1 and 100:1,
and occasionally 500:1. In worm-and-gear sets, where the worm's helix
angle is large, the sliding action between teeth can be considerable,
and the resulting frictional loss causes the efficiency of the drive to
be usually less than 90 percent, sometimes less than 50 percent, which
is far less than other types of gears.
73,
Jon Pearl - W4ABC
www.w4abc.com
On 4/3/2014 3:32 PM, Patrick Greenlee wrote:
> Worm drives with a greater ratio than "the critical ratio" will not
> motor back and freewheel. They "Park" wherever they are left. This is
> a safe situation. Worms with lesser ratios will freewheel and can
> cause injury, death, or at least mess up your tower when it rockets to
> earth. Now if one of the mechanical gurus will please step up and
> remind us of the critical ratio please.
>
> Patrick NJ5G
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